(a) Field of the Invention
The present invention relates to an automatic transmission for vehicles, and more particularly, to a shift control method for an automatic transmission.
(b) Description of the Related Art
In automatic transmissions used for vehicles, a shift control system performs control to realize automatic shifting into different speeds and shift ranges according to various factors including throttle opening, vehicle speed and load, and several engine and driving conditions detected through a plurality of sensors. That is, based on such factors, the shift control system controls a plurality of solenoid valves of a hydraulic control system such that hydraulic flow in the hydraulic control system is controlled, resulting in the shifting of the transmission into various speeds and shift ranges.
In more detail, when the driver manipulates a shift lever to a particular shift range, a manual valve of the hydraulic control system undergoes port conversion as a result of the manual valve being indexed with the shift lever. By this operation, hydraulic pressure supplied from a hydraulic pump selectively engages a plurality of friction elements of a gearshift mechanism according to the duty control of the solenoid valves, thereby realizing shifting into the desired shift range.
When designing such an automatic transmission, power performance is considered to be important in a load range higher than a medium engine load, while fuel consumption ratio is considered to be important in a load range lower than the medium engine load. The medium engine load is set on the basis of 50% of a throttle opening.
That is, the automatic transmission has an economy mode and a power mode. In the economy mode, a shift pattern is designed such that an upshifting operation is quickly realized to improve the fuel consumption ratio. In a power mode, a shift pattern is designed such that an upshifting point is moved to a high speed side so that engine torque can be increased.
The economy and power modes are selected by a user manipulating a pattern select switch.
However, since such shift patterns are fixed in a memory, it is impossible to set an optimum shifting point according to variations in engine torque, torque converter state, and running resistance. That is, the shift patterns cannot be flexibly varied according to these variations.
The present invention has been made in an effort to solve the above problems.
It is an objective of the present invention to provide a shift control method for an automatic transmission that detects a driver""s desire and a vehicle""s running condition and determines a shifting point which can realize optimum fuel consumption ratio and power performance.
To achieve the above objective, the present invention provides a shift control method for an automatic transmission comprising the steps of determining if a vehicle is running on a level road according to a signal from a drive state detector, selecting one of an economy or a power mode according to a current throttle opening ratio, setting an upshifting point of the selected mode, and determining a speed ratio according to the upshifting point of the selected mode and outputting a corresponding control signal to a drive unit.
The step of determining if a vehicle is running on a level road further comprises the steps of determining a gradient value according to a signal from the drive state detector, determining if the gradient value is in a preset standard gradient range, and determining that the vehicle runs on the level road when the gradient value is in the preset standard gradient range or runs on the slope when the gradient value is not in the preset standard gradient range.
Preferably, the preset standard gradient range is xe2x88x923.5-5%.
The step of selecting one of an economy or a power mode further comprises the steps of detecting the current throttle opening ratio, determining if the detected throttle opening ratio is less than a predetermined standard throttle opening ratio, and selecting the economy mode when the detected throttle opening ratio is less than the predetermined standard throttle opening ratio and the power mode when the detected throttle opening ratio is not less than the predetermined standard throttle opening ratio.
Preferably, the standard throttle opening ratio is about 50%.
The method may further comprise the step of setting an upshifting point of a slope running mode when the vehicle runs on the slope, setting a speed ratio according to the upshifting point of the slope running mode and outputting a control signal to the drive unit after a speed ratio is set.
When setting the upshifting point of the slope running mode, a point where a driving force line of a current n-speed intersects a driving force line of an n+1 speed becomes the upshifting point.
When the upshifting point of the economy mode is set, a point where a fuel consumption line of a current xe2x80x9cnxe2x80x9d speed intersects a fuel consumption line of an xe2x80x9cn+1xe2x80x9d speed ratio becomes the upshifting point.
When upshifting point of the power mode is set, a point where a torque line of a current xe2x80x9cnxe2x80x9d speed ratio intersects a torque line of an xe2x80x9cn+1xe2x80x9d speed ratio becomes the upshifting point.
The speed ratio is determined according to the following equation:
No=No_Economy+(No_Powerxe2x88x92No_Economy)xc3x97SUM(PFC)
where No is a speed ratio;
No_Economy is a current speed ratio in an economy mode shifting pattern;
No_Power is a current speed ratio in a power mode shifting pattern; and
SUM(PFC) is the sum of power factor counters.
The SUM(PFC) satisfies the following condition:
0 less than SUM(PFT) less than 1
When a shift control level is in one of neutral N, park P, and reverse R ranges, the SUM(PFC) is set at xe2x80x9c0.xe2x80x9d